1. Field of the Invention
The present invention relates to a digital-to-analog converter, and more particularly, to a current-steering digital-to-analog converter.
2. Description of the Prior Art
Digital-to-analog converters (DAC) are one of the most important electronic devices within a communication apparatus. DACs can be classified into a number of varieties. A current-steering digital-to-analog converter is a common high-speed digital-to-analog converter, and the basic concept is to control the switching device to conduct current to charge the output terminal. Modern semiconductor processes, however, have a trend of utilizing lower supply voltage, which results in the operating voltage range of the transistor becoming narrower. Accordingly, the transistor may operate in an incorrect operation region. Please refer to FIG. 1 in conjunction with FIG. 2. FIG. 1 is a diagram illustrating a prior art current-steering digital-to-analog converter 10, and FIG. 2 is a diagram illustrating the output voltage Vout of a single output terminal of the current-steering digital-to-analog converter 10 shown in FIG. 1. When the supply voltage Vdd=1.8V, the output voltage Vout at the single output terminal will be operated in the range of Vdd−1.25 to Vdd+1.25. Accordingly, the lowest output voltage at the source terminal of the transistor M1 is 0.55V, therefore the output stage transistor M2, which provides the current I, is unable to operate under the saturation region and drops into the triode region. Therefore, the output current will be decreased and cause the distortion of the differential output signal.